1. Field of the Invention
The present invention relates to a thermosensitive recording material comprising as the main components a leuco dye serving as a coloring agent and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto. In addition, the present invention also relates to an aromatic carboxylic acid compound useful as the color developer for use in the thermosensitive recording material, and the production method of the above-mentioned compound.
2. Discussion of Background
Generally, a thermosensitive recording material comprises a support and a thermosensitive coloring layer formed thereon, which comprises as the main components a colorless or light colored electron-donating dye precursor, and an electron-accepting color developer. These dye precursor and color developer are caused to react instantaneously upon the application of heat thereto to produce recorded images, for instance, using a thermal head, heat pen or laser beam, as disclosed in Japanese Patent Publications 43-4160 and 45-14039.
A thermosensitive recording material is used in a wide variety of fields, for example, as the recording material for an electronic computer, facsimile apparatus, ticket vending apparatus, label printer, and recorder because it has the advantages that recording can be achieved using a relatively simple apparatus, maintenance is simple, and there is no noise development.
The above-mentioned thermosensitive recording material employing such an electron-donating dye precursor and an electron-accepting color developer has excellent characteristics such as good appearance and nice touch and is capable of producing images with high coloring density, but also has the disadvantage that the image preservability thereof is poor. To be more specific, when image areas formed in the thermosensitive recording material come in contact with plastics such as polyvinyl chloride, images are decolorized by plasticizers and additives contained in such plastics, or when the image areas come in contact with chemicals contained in foods or cosmetics, such image areas are easily decolorized or the background is easily colored.
To improve the preservation stability of the image recorded in the thermosensitive recording material, it is proposed to use color developers with high reliability. For instance, the use of a phenolsulfone compound as the color developer is disclosed in Japanese Laid-Open Patent Applications 58-82788 and 60-13852; a metallic salt of benzoic acid, disclosed in Japanese Laid-Open Patent Application 61-47292; and a substituted salicylic acid compound, disclosed in Japanese Laid-Open Patent Application 62-169681. However, even though the aforementioned compounds are used as the color developers, the fastness to plasticizers of the image areas formed in the thermosensitive recording material is not sufficient.
Furthermore, the use of a monoester of nitrophthalic acid is proposed in Japanese Laid-Open Patent Application 62-80089, but this compound is unsatisfactory in terms of the fastness to plasticizers of the image areas formed in the thermosensitive recording material. In addition, a sulfonylurea-group-containing compound is used as the color developer in Japanese Laid-Open Patent Application 6-255262. When this type of compound is used as the color developer, the coloring sensitivity extremely deteriorates although the fastness to plasticizers of the image area is improved.
In Japanese Laid-Open Patent Application 9-267566, it is proposed to use as the color developer a compound excluded in the present invention, that is, a compound (B) as will be described in detail later, and metallic salts thereof. However, these compounds have the shortcomings that the image area formed in the thermosensitive recording material is readily decolorized under the circumstances of high temperature.
By the way, it is believed that synthesis of a nitrophthalamide dimer, in particular, a 3-nitrophthalamide dimer can be achieved by amidation reaction of 3-nitrophthalic acid by use of an amine compound. A reaction between 3-nitrophthalic anhydride and an amine compound is reported, for example, in IDDIAN J. CHEM., VOL. 7, 634-635 (1969) and J. Am. Chem. Soc., Vol. 57, 1064-1065 (1935). According to those references, as shown in the following reaction schemes (1) and (2), the amidation reaction of 3-nitrophthalic anhydride takes place at the 1-position of 3-nitrophthalic acid due to steric hindrance when an aromatic primary amine such as aniline is used as the amine compound; while the amidation reaction is carried out at the 2-position thereof when ammonium is used as the amine compound because this reaction is less subjected to steric hindrance.
[Reaction Schemes] ##STR1##
It is considered that the difference in reactivity as shown in the reaction schemes (1) and (2) results from the steric hindrance caused when carbonyl group in 3-nitrophthalic anhydride is attacked by an amine compound.
When 3-nitrophthalamide dimers of the following formulas (V) and (VI) were separately synthesized by allowing 3-nitrophthalic anhydride to react with a diamine compound under the same conditions as stated in the aforementioned references, there was obtained a mixture of the isomers of formulas (V) and (VI). ##STR2##
wherein Y represents, for example, an alkylene group having 2 to 12 carbon atoms, a xylylene group, ##STR3##
In this case, although those two 3-nitrophthalamide dimers (V) and (VI) can be isolated from each other, each compound cannot be efficiently obtained as a pure product because the separation is very difficult.
Further, the aforementioned 3-nitrophthalamide dimers (V) and (VI) can be independently synthesized in accordance with the following reaction schemes (3) and (4):
[Reaction Schemes] ##STR4##
However, the above-mentioned reaction schemes (3) and (4) are very long and include complicated reaction steps, so that they are not practical. There is increasing a demand for the establishment of efficient synthesis method.